Basically, I'm looking for a system which have trading(in a demo account), book keeping, profit/loss and risk calculation capability across different asset classes such as bond, repo, equity, foreign exchange, derivatives etc.

The OpenGamma platform (www.opengamma.com) is probably the most comprehensive trading and risk analytics offer in the open source world. It supports the interest rate, forex and equity products you mentioned. It contains also a strong quantitative finance library.